Ultrahigh cycle fatigue of nanocrystalline NiTi tubes for elastocaloric cooling

Elastocaloric cooling using superelastic NiTi shape memory alloy with reversible phase transformation is a promising environmentally friendly technology, but the limited fatigue life of conventional coarse-grained NiTi remains a crucial bottleneck. Here, we report the ultrahigh fatigue life of nanocrystalline NiTi tubes, exceeding 120 million cycles under a compressive stress of 800 MPa. The NiTi tubes demonstrate stable cyclic stress-strain responses and a stable adiabatic temperature drop of 6.6 K in the lifespan. The material coefficient of performance increases from the initial 8.8 to 11.6 of the 10⁸th compressive cycle. The high resistance to nucleation and growth of compression-parallel cracks results in the ultrahigh fatigue life of the tubes. Our research shows the great potentials of nanocrystalline NiTi tubes with both stable thermomechanical properties and long compressive fatigue lives for reliable elastocaloric cooling.